This invention relates to an apparatus for measuring the velocity of moving material, e.g. gaseous, liquid or solid substances. More particularly, this invention relates to an apparatus employing the laser doppler effect for measuring the velocity of moving media.
Laser velocimeters are known which employ the doppler effect to measure the velocity of moving fluid at a measuring point. Such laser doppler velocimeters have the potential advantage over known mechanical devices for velocity component measurements of permitting such measurements without significantly disturbing the flow characteristics of the fluid being measured. Other advantages of laser velocimeters over mechanical devices are the relatively high speed of the measurement process and the ability of such devices to perform measurements in relatively inaccessible locations.
Although laser velocimeters using pulsed laser sources have been suggested in the literature, known laser velocimeters typically employ a continuous wave laser source. The coherence length of the light generator in such a source is selected to be relatively long, e.g. of the order of several centimeters or more, in order to eliminate the requirement of exact path length equality of two related wavefronts from source to point of optical interference. In such systems, the use of a short coherence length continuous wave or pulsed laser, e.g. an injection laser diode, is precluded due to the fact that interference of beams from such a source cannot be predictably achieved due to the lack of substantially equal path lengths for related beams. Other limitations inherent in known continuous wave laser velocimeters are the relatively large physical size necessitated by the relatively large lasers and associated optics employed, and their relative instability in use, which necessitates frequent readjustment using relatively sophisticated alignment techiques.